In addition to the requirement that source rock exists for the generation of hydrocarbons, and that reservoir rock exists for the storage and production of the generated hydrocarbons, traps must also exist to trap, or seal, the hydrocarbon in place forming a hydrocarbon reservoir.
The fluids of the subsurface migrate according to density. As previously discussed, the dominant fluids present or potentially present are hydrocarbon gas, hydrocarbon liquid, and saltwater. Since the hydrocarbons are less dense than the saltwater, they will tend to migrate upward to the surface, displacing the heavier water down elevation. These fluids will continue to migrate until they encounter impermeable rock, which will serve as a reservoir “seal” or “trap.” These impermeable rocks serving as reservoir seals, of which shale’s are among the most common, are referred to as confining beds or cap rocks. Traps exist because of variations in characteristics of rocks of the subsurface. If impermeable rock does not exist, the hydrocarbons will migrate to the surface and dissipate into the environment. In order for a hydrocarbon reservoir to exist, a proper sequence of events must have occurred in geologic time.
Traps can be classified as:
is a shifting or alteration in the horizontal formations of the earth’s crust. The alteration is caused by the physical processes of plate tectonics, continental drift, earthquakes, rifting or the intrusion of salt, shale or serpentine. The intrusion forms faults and folds in the original horizontal formations thus creating the traps necessary for reservoirs Other structures common to hydrocarbon reservoirs are folds and faults
type of Structural Traps:
1) Anticline Traps:
Sedimentary beds are generally deposited in horizontal parallel planes over a geographic region, so that many of these sediments will be of essentially uniform thickness over This trap may exist as a simple fold or as an anticlinal dome. that region. If geologic activity should occur, resulting in the folding of these sediments, the result may be the formation of hydrocarbon reservoirs in anticlinal traps. Two major potential advantages of the anticlinal trap reservoir are the simplicity of the geology and the potential size of the trap and therefore of the hydrocarbon accumulation. The high part of the fold is the anticline, and the low part of the fold is the syncline. Since the hydrocarbons are the less dense of the subsurface fluids, they will tend to migrate to the high part of the fold. Consider the hydrocarbon reservoir illustrated in Figure 18. Hydrocarbon reservoir rock,where shale is the cap rock formation of this hydrocarbon reservoir. Sedimentary beds are deposited in a water environment, as indicated by the presence of limestone’s and shale’s. During or after lithification, geologic activity causes folding of the sediments. After folding and lithification, the sandstone has a 100% connate water saturation. Millions of years later, hydrocarbon generated in source rock down elevation from this anticlinal fold is forced from its source rock into the water-saturated, permeable sandstone. Since hydrocarbon is less dense than the water, it begins to migrate up elevation, displacing the heavier water down elevation. As it migrates upward, pressure decreases. At some point in this migration, the reservoir fluid pressure might equal the bubble point pressure of the original hydrocarbon combination. From this point upward, gas is being released from the hydrocarbon. Since the gas is so much less dense than the oil or the water, it will migrate more rapidly toward the top of the anticlinal trap. This process of migration and fluid separation according to density may continue over millions of years in geologic time, until finally, equilibrium is achieved as the hydrocarbon fluids accumulate within the trap formed by the impermeable shale cap rock When this condition of equilibrium is finally achieved, there will be a gas zone (gas cap) on top of an oil zone and then a water zone beneath the oil zone.
2) Fault Traps :
Fault implies fracturing of rock and relative motion across the fracture surface. Consider a possible sequence of geologic events that, in geologic time, . Sedimentary beds are deposited in a water environment, as indicated by the presence of shale’s and limestone’s. During or after lithification,geologic events result in uplift of these original horizontal sediments, and fracturing and tilting above sea level, so that the surface rocks are exposed to erosion. During uplift, the rocks are fractured and slippage occurs along the fault plane.This brings the shale across the fault so that it seals the tilted sandstone below the fault. Millions of years later, hydrocarbon generated in its source rock down elevation from the fault is forced into the connate water-saturated sandstone. Since the hydrocarbon is less dense than the water, it will migrate up elevation, displacing the heavier water down elevation. This upward migration will continue until it reaches the fault and is trapped by the impermeable shale. If the faulting had not occurred, the hydrocarbon would have continued to migrate upward until it was dissipated at the surface into the environment. Since faulting occurred, the shale provides the necessary seal, resulting in the existence of the hydrocarbon reservoir.Notice that, in this example, if slippage had occurred to a greater extent, there would have been flow into the permeable sandstone above the fault. The hydrocarbon would have been lost to the surface, and no reservoir would have been formed.This situation illustrates the significance of geologic probability.
What is the probability that the relative motion across the fault would have resulted in a reservoir seal being formed?Geologic events must occur in the proper sequence, resulting in the proper geologic conditions for a reservoir to exist. The North Sea hydrocarbon environment is an excellent example of the
significance of this geologic probability. Of the hydrocarbon generated in the source rock of the North Sea, it is estimated that less than 10% was trapped. Over 90% of the hydrocarbon was lost back to the surface in geologic time and dissipated into the environment because traps were not present. Fault traps leading to the presence of hydrocarbon reservoirs are often difficult to define because of the complexity of the geology.
3) Salt Dome Traps:
Consider the salt dome geologic system illustrated in Figure and a possible sequence of geologic events that could lead to the formation of this salt dome environment. A major portion of a continental plate was below sea level at a point in geologic history. Due to geologic events, this region rose above sea level, trapping inland a salt water sea. As geologic time passed,the climate changed to a desert environment. This event could have resulted from movement of the continental plate near tothe equator. In this arid desert environment, water evaporated from the salt water sea, leaving the salt residue on the dry seabed. As millions of years passed in the desert environment,sand blew over the salt to cover and protect the salt sediment.Later geologic events resulted in the sinking of the region below sea level, followed by tens of millions of years of sedimentation in the resulting water environment. As time passed, lithification occurred. The desert sand became sandstone, and the salt became rock salt (sedimentary salt).
After lithification, this salt bed was impermeable. It also had two properties significantly different from typical shale, sandstone or limestone:
- It was less dense, with a measurably smaller specific weight.
- At subsurface overburden pressures and subsurface
temperatures, the rock salt was a plastic solid (it was highly deformable).
The combination of this lesser density and plasticity resulted in a buoyant effect if flow possibilities existed. Geologic events caused fracturing of overlying confining rocks. The salt, forced upward by the overburden pressures, began to flow plastically back to the surface, intruding into the overlying rock structures
to lift, deform, and fracture them. The intruding salt was solid,yet geologically deformable. It might intrude at an average rate of only 1 inch per 100 years, yet on a geologic time basis, such deformation is highly significant. This rate would result in 10inches in 1,000 years, or 10,000 inches (833 ft) in 1 million years. In a geologic time period of only 10 million years, this salt dome could intrude to a height of over 1.5 mileoverlying structures. Obviously, a vertical subsurface structure1.5 miles high is geologically significant. Since the salt isimpermeable, the region around the perimeter of the salt domeis an ideal geologic environment for hydrocarbon traps. The tendency of the intruding salt to uplift the rocks as it intrudesenhances the separation of the less dense oil from the more dense salt water by reducing the area of the oil-water contact.The fracturing of surrounding rocks due to the intruding salt and the lifting of the rocks above the salt dome also provide an environment for the existence of fault traps and anticlinal traps in addition to the salt dome traps around the perimeter of the
dome. A salt dome region, therefore, is an excellent geologic environment for all three types of traps discussed so far.An excellent example of a salt dome trap is Spindle top near Beaumont, Texas. The first major discovery and resultant initial oil boom at Spindle top occurred in 1901. Through the 1890s Patillo Higgins had promoted drilling for oil outside Beaumont. He concluded that it was an excellent geologic environment for hydrocarbon reservoirs, because he noted a location near Beaumont where the surface elevation was 15 ft higher than the surrounding land. This rise was a circle approximately 1 mile in diameter. He concluded that this indicated high points in the underlying geology. In 1901, Captain Anthony Lucas drilled a wildcat well at this location, resulting in the Spindle top discovery. Future drilling confirmed that this reservoir existed as an anticlinal dome trap, with the dome created by the uplift of overlying rocks by an intruding salt dome in the subsurface, creating the surface indication of what the subsurface geology might be.The second oil boom at Spindle top began in the mid-1920s.
When further wells were drilled, it was discovered that fault trap and salt dome trap reservoirs existed around the circumference of the salt dome. The drilling pattern for the wells drilled during this later activity was almost a perfect circle as these circumferential reservoirs were developed.
The stratigraphic trap is a change in the lithology of the rock sequence. This change is caused by erosional forces or changes in rock type within a limited areal extent. An unconformity is an erosional feature where a portion of the geological sequence is eroded and an impermeable rock is deposited on top of a porous formation. The process of erosion will enhance or create the porosity and permeability necessary for the existence of a petroleum reservoir. Other stratigraphic trapping include channel sand deposits surrounded by shale, growth of limestone reefs and the formation of barrier islands or sand bars along the ancient shoreline.
Classification of stratigraphic trap:
-Primary Stratigraphic Traps:
These traps result from deposition of elastic or chemical materials. Shoestring sands, lenses, sand patches, bars, channel fillings, facies changes, strand-line (shoreline) deposits, coquinas, and weathered or reworked igneous materials are classified as elastic sedimentary deposits and can result in stratigraphic traps. An ancient sand-filled stream channel meander has cut into older south-dipping shales and created a perfect stratigraphic trap.
The shale plug served as the seal for reservoirs within a west-plunging structural nose. Hydrocarbons are trapped in the truncated up dip portions of the reservoirs. Organic reefs or biohenns and biostromes are the primary chemical stratigmphic traps; they are built by organisms and are foreign bodies to the surrounding deposits .The Strawn and Cisco-Canyon series are limestone reefs that have had younger
the seal. Differential compaction of the thicker shales on the Type of stratigraphic trap : flanks of the reef as compared with the thinner shale at the crest has created structural closure in younger overlying formations. Hydrocarbon accumulations have occurred in the Cisco and Fuller formations as a result of this differential compaction. Additional traps in other reservoirs arc the result of up dip permeability and porosity barriers and are either primary or secondary stratigraphic traps.
Secondary Stratigraphic Traps:
Traps of this type were formed after the deposition of the reservoir rock by erosion and/or alteration of a portion of the reservoir rock through solution or chemical replacement. Secondary tratigmphic traps actually should fall into the combination-trap classification because most are associated with or are the result of structural relief during some stage of development of porosity and permeability or limitation of the reservoir rock. However, many of the so-called typical “stratigraphic traps” fall into this category, and it is felt that it would be impossible tochange the historical usage of this term. Therefore, secondary stratigraphic traps are defined for this discussion as those traps created after deposition and having limitations caused by lithology changes.
Erosion creates a major part of these through truncation of the reservoir rock. On-lap deposition (when the water is encroaching landward), off-lap deposition (when the water is regressing), and the chemical alteration of limestone result in many secondary stratigraphic traps) It is a truncation of the Woodbine formation as it approaches the regional Sabine uplift. A certain amount of leaching of the cementing material by waters over the unconformity has resulted in increased porosity and permeability in the field as compared with similar Woodbine sands in the deeper portions of the East Texas basin.